As a Roblox player, it is currently impossible to change the order of layers in the avatar editor.
Consider the following, I use an avatar with overalls, because these overalls are deemed as a “Bottoms” item, all shirts render ontop of it.
Anyone who has worn overalls irl can tell you, you wouldn’t put a T-Shirt ontop of a pair of overalls
Current behaviour:
Expected behaviour if I was able to change the layers.
If Roblox were to address this issue it would give us more creativity with avatars by letting us creating more realistic (or less realistic depending on who you are) layer patterns. Some people may just want to wear trousers over shirts which is currently impossible.